1. Field of the Invention
The present invention relates to a transmission/reflection type (hereinafter xe2x80x9ctransflectivexe2x80x9d) color liquid crystal display device having both reflection and transmission functions. A transflective color liquid crystal display device operates in a reflection display mode using ambient light from around the device under bright environments and in a transmission display mode using light from the backlight under dark environments. The liquid crystal display device of the present invention is suitable for use in various applications, including OA equipment such as word processors and personal computers, personal digital assistants such as electronic organizers, and camcorders with a liquid crystal monitor.
2. Description of the Background Art
A transflective color liquid crystal display device in portable equipment, etc., has a reduced power consumption and is capable of operating in a transmission display mode using a backlight, whereby it is suitable for use under various environments, and the use thereof as a display device in portable equipment, etc., is becoming widespread.
A conventional transflective color liquid crystal display device includes a color filter layered on a reflection film that is formed on a reverse-side substrate for producing a color display, and the reflection film is a transflective reflection film including light-transmitting openings in some portions thereof.
The color filter and the reflection film are formed within the liquid crystal panel so as to prevent the display color saturation from lowering due to the parallax caused by the substrate thickness, and a color filter with a high transmittance is employed in order to gain brightness in a reflection mode.
Other semi-transmissive reflection films include half mirror films obtained by thinning a metal such as aluminum or silver, those obtained by patterning a metal through an etching process so that portions thereof where the metal is left unetched are used in a reflection mode while other portions where the metal is removed are used in a transmission mode, and those utilizing interference which are obtained by layering dielectric layers of different refractive indices on one another.
One such liquid crystal display device is disclosed in, for example, Japanese Laid-Open Patent Publication No. 11-052366, where a color filter is layered on a reflection film that is formed on a reverse-side substrate, and the reflection film includes light-transmitting openings in some portions thereof that are opposing pixels of the color filter.
Japanese Laid-Open Patent Publication No. 11-183892 proposes a liquid crystal display device in which openings are provided in a color filter on a front-side substrate, a reflection film is provided on the inner surface of a backside substrate in positions corresponding to the openings of the color filter, and a semi-transmissive reflector is provided on the reverse surface of the back-side substrate. It is stated that with this arrangement, it is possible to display an image through color pixels with a high brightness in a reflection display mode by using colored light that passes through portions of the color filter other than the openings and is reflected by the semi-transmissive reflector and non-colored light of a high brightness that passes through the openings of the color filter and is reflected by the reflection film, while it is possible to display an image through color pixels with a high contrast in a transmission display mode by outputting, to the front side of the device, only the colored light that has passed through the portions of the color filter other than the openings.
However, if a color filter with a high transmittance for gaining brightness in a reflection mode is employed in the liquid crystal display device described in Japanese Laid-Open Patent Publication No. 11-052366, although the display color saturation is high in a reflection display mode since light passes through the color filter twice, the display color saturation decreases considerably in a transmission display mode since light passes through the color filter only once. Moreover, if a color filter with a high saturation (low transmittance) is employed in an attempt to increase the display color saturation in a transmission display mode, the brightness in a reflection display mode decreases considerably, thereby lowering the visibility significantly.
The liquid crystal display device described in Japanese Laid-Open Patent Publication No. 11-183892 requires two reflection films. Moreover, reflected light from the reflection film on the inner surface of the back-side substrate and reflected light from the transflective reflector on the reverse surface of the back-side substrate are both used, thereby causing problems such as a decrease in the color purity due to the parallax caused by the presence of the back-side substrate.
The present inventors have found that when any of the layers of a transflective color liquid crystal display device provided on the viewer side with respect to the reflection film (e.g., a viewer-side substrate, an overcoat film, a liquid crystal layer, an alignment film, etc.) is colored in a particular color, a color displayed image as a whole is colored in the particular color not only in a reflection display mode but also in a transmission display mode, thereby causing a decrease in the color reproducibility, which has not been significant in a black and white display.
Moreover, in recent years, there is an increasing demand for reducing the thickness/weight of a transflective color liquid crystal display device. Accordingly, there has been an active research for commercializing a display device using a plastic substrate instead of using a glass substrate as in the prior art.
However, when a transflective color liquid crystal display device is produced by using a plastic substrate instead of using a glass substrate as in the prior art, a colorless and transparent plastic substrate gets yellowish in some cases through a production process such as heating during a color filter deposition, a transparent electrode film deposition or an alignment film deposition. In such a case, the color displayed image as a whole gets yellowish not only in a reflection display mode but also in a transmission display mode, thereby causing a decrease in the color reproducibility.
A liquid crystal display device of the present invention includes: a first substrate; a second substrate opposing the first substrate; a liquid crystal layer interposed between the first substrate and the second substrate; a reflection film formed on one side of the second substrate that is closer to the liquid crystal layer; and a color filter formed on the reflection film, wherein: a plurality of pixel regions are arranged in a matrix pattern, each of the pixel regions including a reflection region where light coming from the first substrate side is reflected by the reflection film back to the first substrate side, and a transmission region where light coming from the second substrate side is transmitted to the first substrate side; and the color filter includes an opening in the reflection region.
Thus, in the liquid crystal display device of the present invention, the opening of the color filter is provided in the reflection region of each pixel so that the opening does not overlap with the transmission region.
With the liquid crystal display device of the present invention, in a transmission display mode where a light source provided on the back side of the liquid crystal display device is used, light that is transmitted through the transmission region passes through the color filter to the outside, thereby obtaining a bright display with a satisfactory display color saturation. Desired characteristics can be obtained by adjusting the luminance of the light source, the area and shape of the transmission region on the second substrate, and the saturation, transmittance and thickness of the color filter.
The transmission region is usually a region where the reflection film is not formed. However, the term xe2x80x9ctransmission regionxe2x80x9d may include a region of the reflection film where the thickness thereof is reduced so that light from the light source provided on the back side of the liquid crystal display device is transmitted through the region with a transmittance of 90% or more, and preferably 95% or more. The term xe2x80x9creflection regionxe2x80x9d includes not only a region where light from the first substrate side (viewer side) is reflected in total reflection (reflectance: 100%), but also a region where the light from the first substrate side is reflected with a reflectance of 90% or more, and preferably 95% or more, with a portion of the light being transmitted.
In a reflection display mode where ambient light is used, light entering the liquid crystal display device from the front side (viewer side) passes through the color filter or the color filter openings, is reflected by the reflection regions of the reflection film, and passes again through the color filter or the color filter openings to the outside, whereby combined output light of non-colored output light and colored output light is obtained, thus realizing a bright display. The brightness and saturation of the output light can be adjusted by adjusting, as necessary, the characteristics of the color filter and the area and shape of the color filter openings. By enlarging the color filter openings, a color filter having a high color purity can be employed.
In the liquid crystal display device of the present invention, it is preferred that the color filter includes a plurality of openings in the reflection region. In this way, non-colored output light is output to the first substrate side while being dispersed across each pixel in a reflection display mode, whereby bright regions are dispersed across each pixel, thus improving the visibility.
In the liquid crystal display device of the present invention, it is preferred that the reflection film is a diffused reflection film whose surface on the liquid crystal layer side has irregularities. In this way, in a reflection display mode, reflected light from the reflection film passes through the color filter to the first substrate side while being diffused, whereby the brightness and the saturation of the combined output light are made uniform across each pixel, thus improving the visibility.
In the liquid crystal display device of the present invention, it is preferred that an area proportion of the transmission region with respect to the pixel region is equal to or greater than 10% and less than or equal to 50%, and an area proportion of the opening with respect to the reflection region is equal to or greater than 5% and less than or equal to 30%. In this way, it is possible to realize a transflective color liquid crystal display device that is satisfactory for practical use in a reflection display mode and in a transmission display mode. Specifically, a sufficient brightness can be ensured in a transmission display mode by setting the area proportion of the region where the reflection film is not formed (i.e., the transmission region) to be 10% or more, and a sufficient brightness can be ensured in a reflection display mode by setting it to be 50% or less. On the other hand, a sufficient brightness can be ensured in a reflection display mode by setting the color filter opening proportion to be 5% or more, and a sufficient color area can be ensured in a reflection display mode, whereby different colors can be distinguished from one another, by setting it to be 30% or less. In order to perform an evaluation in a reflection display mode, the color filter opening proportion is herein represented in relation to the area of the reflection region. This is because the size of the opening of the color filter significantly influences a reflection display.
In the present specification, a region of a liquid crystal display device corresponding to a xe2x80x9cpixelxe2x80x9d, which is the minimum unit of display, will be referred to as a xe2x80x9cpixel regionxe2x80x9d. In a color liquid crystal display device, for example, R, G and B xe2x80x9cpixel regionsxe2x80x9d correspond to one xe2x80x9cpicture element regionxe2x80x9d. In a passive matrix type liquid crystal display device, a pixel region is defined as a region where one of column electrodes which are arranged in a stripe pattern crosses one of row electrodes which are also arranged in a stripe pattern perpendicular to the column electrodes. In an active matrix type liquid crystal display device, a pixel region is defined by a pixel electrode and a counter electrode which opposes the pixel electrode. In an arrangement with a black matrix, strictly speaking, a pixel region is a portion of each region through which a voltage is applied according to the intended display state that corresponds to an opening of the black matrix.
In the liquid crystal display device of the present invention, it is preferred that the openings of the color filter are filled with a transparent resin whose transmittance is 90% or more. In this way, the step around each color filter opening is eliminated, whereby the liquid crystal molecules rise in a uniform manner in the vicinity of each color filter opening, thus improving the contrast (particularly the contrast in a reflection mode). A flattening film made of an acrylic resin is layered on the surface of the color filer for flattening the surface irregularities of the color filter. Thus, when the color filter openings are shallow, the openings are filled with the flattening film. However, when the color filter openings are deep, unless the openings are filled with a transparent resin, the step around each opening cannot be eliminated by the flattening film, thus leaving a step. Particularly when an STN liquid crystal material is used, the liquid crystal molecules rise in a non-uniform manner in each pixel due to the presence of such a step around each color filter opening, thereby causing significant problems such as a reduction in the contrast in a reflection display mode.
A transparent resin may be of any type as long as the transmittance thereof is higher than that of the color filter. However, in order to ensure a sufficient brightness in a transmission display mode, a high transmittance is preferred. Even if a pigment for tint correction is mixed in the transparent resin, it is desirable to ensure a transmittance of 90% or more, and preferably 95% or more.
In the liquid crystal display device of the present invention, it is preferred that: a plurality of picture element regions are defined, each including a number of the pixel regions of different colors; and in each of the plurality of picture element regions, an area of the opening of the color filter in at least one of the pixel regions of different colors is smaller than an area of the opening of the color filter in each of the other pixel region(s).
With such a liquid crystal display device, if reflected light is colored in a particular color by a layer provided on the viewer side with respect to the reflection film (e.g., the first substrate, the overcoat film, the liquid crystal layer, the alignment film, etc.), the area proportion of a color filter of a hue that is approximately the complementary color to that particular color can be increased relative to those of the color filters of other hues. Therefore, it is possible to realize a reflection color display with a desirable color reproducibility in a reflection display mode.
The liquid crystal display device of the present invention may be such that: the first substrate is a plastic substrate; a plurality of picture element regions are defined, each including three of the pixel regions of red, green and blue; and in each of the plurality of picture element regions, an area of the opening of the color filter in the blue pixel region is smaller than an area of the opening of the color filter in each of the red and green pixel regions.
With such a liquid crystal display device, if a transparent plastic substrate (the first substrate) provided on the viewer side with respect to the reflection film gets yellowish through a production process, the area proportion of the color filter of blue, which is approximately the complementary color to yellow, can be increased relative to those of the red and green color filters. Therefore, in a reflection display mode, the reflected light can be approximately white light in a white display, thereby realizing a reflection color display with a desirable color reproducibility.
In the liquid crystal display device of the present invention, it is preferred that: a plurality of picture element regions are defined, each including a number of the pixel regions of different colors; and in each of the plurality of picture element regions, an area of the transmission region in at least one of the pixel regions of different colors is greater than an area of the transmission region in each of the other pixel region(s).
With such a liquid crystal display device, if transmitted light is colored in a particular color by a light source such as a backlight, or a layer present in the path along which light from the light source travels (e.g., the light guide plate, the first substrate, the second substrate, the liquid crystal layer, etc.), the area proportion of the color filter of a color that is approximately the complementary color to that particular color can be increased relative to those of the color filters of other hues. Therefore, it is possible to realize a transmission color display with a desirable color reproducibility in a transmission display mode.
In the liquid crystal display device of the present invention, it is preferred that: at least one of the first substrate and the second substrate is a plastic substrate; a plurality of pixel regions are defined, each including three of the pixel regions of red, green and blue; and in each of the plurality of pixel regions, an area of the transmission region in the blue pixel region is greater than an area of the transmission region in each of the red and green pixel regions.
With such a liquid crystal display device, if a transparent plastic substrate present in the path along which light from a light source such as a backlight travels gets yellowish through a production process, the area proportion of the color filter of blue, which is approximately the complementary color to yellow, can be increased relative to those of the red and green color filters. Therefore, in a transmission display mode, the transmitted light can be approximately white light in a white display, thereby realizing a transmission color display with a desirable color reproducibility.
In the liquid crystal display device of the present invention, it is preferred that: a plurality of picture element regions are defined, each including a number of the pixel regions of different colors; and in each of the plurality of picture element regions, an area of the opening of the color filter in at least one of the pixel regions of different colors is smaller than an area of the opening of the color filter in each of the other pixel region(s), and an area of the transmission region in at least one of the pixel regions of different colors is greater than an area of the transmission region in each of the other pixel region(s).
With such a liquid crystal display device, if reflected light or transmitted light is colored in a particular color by a layer provided on the viewer side with respect to the reflection film, a light source such as a backlight, a layer present in the path along which light from the light source travels, etc., the area proportion of the color filter of a hue that is approximately the complementary color to that particular color can be increased relative to those of the color filters of other hues. Therefore, it is possible to realize a reflection color display with a desirable color reproducibility in a reflection display mode and in a transmission display mode.
In the liquid crystal display device of the present invention, it is preferred that: at least one of the first substrate and the second substrate is a plastic substrate; a plurality of pixel regions are defined, each including three of the pixel regions of red, green and blue; and in each of the plurality of pixel regions, an area of the opening of the color filter in the blue pixel region is smaller than an area of the opening of the color filter in each of the red and green pixel regions, and an area of the transmission region in the blue pixel region is greater than an area of the transmission region in each of the red and green pixel regions.
With such a liquid crystal display device, if a transparent plastic substrate provided on the viewer side with respect to the reflection film or in the path along which light from a light source such as a backlight travels gets yellowish through a production process, the area proportion of the color filter of blue, which is approximately the complementary color to yellow, can be increased relative to those of the red and green color filters. Therefore, in a reflection display mode and in a transmission display mode, the reflected light and the transmitted light can be approximately white light in a white display, thereby realizing a transmission color display with a desirable color reproducibility.